Flux composition and techniques for use thereof

ABSTRACT

The present invention is directed to a soldering method for joining objects is also provided, comprising the steps of applying a flux composition to at least a portion of one or more of the objects, and joining the objects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/034,932, filed Feb. 25, 2011, incorporated by reference herein.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to soldering techniquesand, more particularly, to flux compositions and techniques for usethereof.

BACKGROUND OF THE INVENTION

Fluxes play an important role in solder-joining electronic components,such as semiconductor devices, onto printed circuit cards or printedcircuit boards (PCBs). Flux is used in a process of flip-chip joining toa substrate that has ball grid arrays (BGA) or land grid arrays (LGA).In a typical process, by way of example, flux is applied onto asubstrate followed by placing a chip onto the flux-applied substrate.Then, the chip-substrate module goes through a reflow process at a hightemperature so as to make solder connections. The subsequently formedflux residue is cleaned (for example, with water) followed by drying themodule. An underfill material is introduced into the gap between thechip and the substrate to maintain the integrity of the flip-chippackage.

As the density of Controlled Collapse Chip Connection (C4) arrays andthe chip size increase, the joining process sometimes produces non-wetsat a corner of the large chip due to the smaller C4 size and the largerwarpage of a substrate. It also becomes more difficult to clean fluxresidue, formed during flip-chip joining, out of the narrowchip-substrate gap of the large chip package.

Non-wets which make electrical open should be avoided, and flux residueoften causes delamination between underfill and chip or betweenunderfill and substrate, resulting in failure of flip-chip packages. Asthe size of solder balls in a chip decrease, slight movement of analigned chip-laminate module during a reflow process can cause non-wets,because a typical high-throughput reflow tool tends to vibrate. Suchnon-wets can increase in the case of multi-chip modules.

Chemical components of a flux or its impurities can decompose orvaporize during reflow to give out vapors, which can cause chips to moveout of position. Highly tacky fluxes can, however, prevent movement ofchips even though vapors are coming out or a reflow furnace vibrates.Also, chemical components of the flux should have a boiling point thatis higher than a typical reflow temperature in order to avoid movementof chips due to boiling. Solvent molecules of the flux, if they do notevaporate upon reflow due to their high boiling points, should be largeenough so as not to diffuse into a substrate outer layer such as soldermask, which is relatively porous because it is a photoresist with silicafillers.

However, after joining, left-over flux and by-products of the reactionbetween flux and solders need to be cleaned, preferably with a low costand environment friendly method because cleaning with organic solvent isnot only harmful to human beings and the environment but also expensive,in regards to both the material itself and the waste treatment process.Therefore, it would be desirable to develop a flux that has sufficienttackiness and a sufficiently high boiling point, while the flux residuecan be cleaned with water.

Many existing fluxes in the industry, however, provide an inadequatejoining capability in case of lead-free solders, give non-wets at thecorner of a chip, leave considerable flux residue onto chip, C4 andsubstrate surfaces after cleaning, diffuse into the substrate outerlayer, and/or cause chip movement during reflow.

By way of example, existing fluxes include disadvantageous aspects suchas being small enough to diffuse into a solder mask layer at a reflowtemperature, resulting in a failure of the semiconductor package, notproviding enough tackiness or viscosity, as well as often leavingresidue after reflow that cannot be cleaned with water.

Accordingly, there is a need for soldering flux compositions that caneffectively manufacture the modern and high-performance semiconductorpackages used for computers, communication devices, home electronics,game consoles, audio/video equipments, automobiles, etc.

SUMMARY OF THE INVENTION

Principles and embodiments of the invention provide a flux compositionand techniques for use thereof. In one aspect of the invention, a fluxcomposition is provided. The flux composition comprises an activator, amedium-viscosity solvent being a polymer, and a high-viscosity solventbeing a copolymer containing a plurality of first monomers and aplurality of second monomers.

In another aspect of the invention, a soldering method for joiningobjects is provided comprising the steps of applying a flux compositionto at least a portion of one or more of the objects, the fluxcomposition comprising an activator, a medium-viscosity solvent being apolymer, and a high-viscosity solvent being a copolymer containing aplurality of first monomers and a plurality of second monomers, andjoining the objects.

In another aspect of the invention, a flux composition is provided. Theflux composition comprises an activator, and a high-viscosity solvent,wherein the high-viscosity solvent comprises a copolymer containing aplurality of first monomers and a plurality of second monomers. In yetanother aspect of the invention, a flux composition is provided. Theflux composition comprises one or more diacid activators, and amedium-viscosity solvent, wherein the medium-viscosity solvent comprisesa polymer.

Further, in yet another aspect of the invention, a flux composition isprovided. The flux composition comprises an activator in a liquid state,and a medium-viscosity solvent, wherein the medium-viscosity solventcomprises a polymer.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a substrate, according to an embodimentof the present invention;

FIG. 2 is a diagram illustrating a flux configuration, according to anembodiment of the present invention;

FIGS. 3A and 3B are diagrams illustrating techniques for soldering asemiconductor device to a substrate, according to an embodiment of thepresent invention;

FIG. 4 is a diagram illustrating a flip-chip package configuration,according to an embodiment of the present invention; and

FIGS. 5A and 5B are flow diagrams illustrating techniques for joiningobjects, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A flux composition suitable for use in solder-joining, for example,electrical components, is disclosed herein. One or more embodiments ofthe invention include a water-soluble highly-tacky flux composition. Asdetailed herein, a flux of one or more embodiments of the invention caninclude properties such as solder joining capability, high tackiness,water cleaning capability, and no boiling at reflow. Tackiness isrelated to and indicated by viscosity; namely, the more viscous aliquid, the tackier the liquid. Because viscosity of a solvent is to beappreciated by one skilled in the art, viscosity is used accordinglyherein.

The composition of a flux, as detailed herein, includes at least anactivator and at least one solvent. Accordingly, such a flux compositioncan include a fluxing agent (also referred to herein as an activator)that includes multi-acids that contain two or more carboxylic acidgroups. One or more embodiments of the invention include usingmulti-acids such as oxalic acid, malonic acid, succinic acid, glutaricacid, adipic acid, pimelic acid, suberic acid, diglycolic acid,diethylenetriamine pentaacetic acid, tartaric acid and poly(acrylicacid). In one or more embodiments of the invention, another component ofsuch a flux includes a medium-viscosity solvent that is employed tocontrol flux application and water cleaning. Yet another component, inone or more embodiments of the invention, can include a high-viscositysolvent to further enhance flux tackiness.

Additionally, in one or more embodiments of the invention, a smallamount of water (approximately 1% by total weight) can be present sincethese solvents can absorb moisture from the air.

By way of example, in one or more embodiments of the invention, amedium-viscosity solvent can include glycerol ethoxylate and/or glycerolethoxylate-propoxylate copolymers, while a high-viscosity solvent caninclude ethylene glycol-propylene glycol copolymers. Both glycerolethoxylate and glycerol ethoxylate-propoxylate copolymers have threehydroxyl end groups, while ethylene glycol-propylene glycol copolymershave two hydroxyl end groups. These hydroxyl end groups can enhancesolubility in water as well as viscosity and tackiness. The solventsalso help dissolve activators to create a homogeneous solution referredto as flux. It should be appreciated by one skilled in the art thatother known high-viscosity and tacky solvents, for example, can also beused. Flux, as detailed herein, is soluble in water to have the fluxresidue cleaned with water, and it remains a homogeneous solution fromroom temperature to a reflow temperature. Homopolymers such aspoly(ethylene glycol) and poly(propylene glycol) are not adequate to behigh-viscosity solvents because they are insoluble in water, they cannotdissolve activators in a solid state and/or their viscosities are toolow.

As described herein, however, copolymers can be high-viscosity solventsfor the fluxes in one or more embodiments of the invention. The firstand second monomers of such copolymers can be in a random or blocksequence to form random copolymer or block copolymer. Additionally, oneor more embodiments of the invention can also include highly viscous andtacky multi-acids, which can play a dual role of an activator and atackiness enhancer. Such a multi-acid can include, for example,poly(ethylene glycol)-diacid.

In a certain type of flip-chip package, a highly tacky and viscous fluxis not required while an effective activator is helpful in making theintended solder connections. Thus, one or more embodiments of theinvention include a flux composition that includes one or moreactivators from the selected diacids (for example, glutaric acid,pimelic acid, tartaric acid, or mixtures thereof) and a medium-viscositysolvent, wherein the medium-viscosity solvent comprises a polymer withhydroxyl end groups.

As described herein, one or more embodiments of the invention can alsoinclude soldering techniques for joining objects. A flux composition,such as one detailed herein, is applied to at least a portion of one ormore of the objects including chips and a substrate. As noted above anddescribed further herein, the flux composition can include a fluxingagent (activator) comprising multi-acids, a medium viscosity and tackyagent (solvent 1) and a high viscosity and tacky agent (solvent 2). Theobjects can be joined at a high temperature. Additionally, flux residueformed during the joining process is removed with water so that nofailure occurs in the microelectronic package product due to uncleanedflux residue that can include flux itself and flux-solder reactionbyproducts. Flux tackiness is high enough to prevent slight movement ofchips from a substrate during the reflow process so as to make thechip-substrate interconnections misaligned.

By way of example and not limitation, one or more embodiments of theinvention can include the composition and use of fluxes such as thefollowing examples. Flux A includes 5-12 weight % (of total compositionweight) glutaric acid as an activator, 15-75 weight % glycerolethoxylate as a medium viscosity solvent, and 20-80 weight % ethyleneglycol-propylene glycol random-copolymer as a high viscosity solvent.Flux B includes 5-12 weight % glutaric acid as an activator, 15-75weight % glycerol ethoxylate as a medium viscosity solvent, and 20-80weight % ethylene glycol-propylene glycol-ethylene glycol blockcopolymer as a high viscosity solvent. Flux C includes 10-20 weight %poly(ethylene glycol)-diacids as an activator and a viscous liquid, 0-90weight % glycerol ethoxylate, and 0-90 weight % ethyleneglycol-propylene glycol random copolymer. Flux D includes 5-12 weight %glutaric acid, 5-15 weight % poly(ethylene glycol) diacids, 0-88 weight% glycerol ethoxylate, and 0-90 weight % ethylene glycol-propyleneglycol random copolymer. Flux E includes 6-12 weight % glutaric acid asan activator, 15-70 weight % glycerol ethoxylate as a medium viscositysolvent, 20-75 weight % ethylene glycol-propylene glycolrandom-copolymer as a high viscosity solvent, and 4-10 weight %multi-amines. Flux F includes 10-20 weight % poly(ethyleneglycol)-diacids as an activator and a viscous liquid, 0-86 weight %glycerol ethoxylate, 0-86 weight % ethylene glycol-propylene glycolrandom copolymer, and 4-10% multi-amines. Multi-amines, as detailedherein, should be soluble in water and can include, for example,tetrakis(4-hydroxypropyl)ethylenediamine (Quadrol®) andtetrakis(4-hydroxyethyl)ethylenediamine.

In one or more embodiments of the invention, the activators can includeketo acid that can be selected from levulinic acid, acetyl butyric acid,2-acetylbenzoic acid, 2-acetyloxybenzoic acid, 2-ketobutyric acid,acetoxyacetic acid, and pyruvic acid or mixtures thereof. Thus, Flux Gcan include 5-15% keto acid as an activator, 15-75 weight % glycerolethoxylate as a medium viscosity solvent, and 20-80 weight % ethyleneglycol-propylene glycol random-copolymer as a high viscosity solvent.

One or more embodiments of the invention also include a flux compositionthat includes one or more activators from the selected diacids (forexample, glutaric acid, pimelic acid, tartaric acid, or mixturesthereof) and a medium-viscosity solvent, wherein the medium-viscositysolvent includes a polymer with hydroxyl end groups. Accordingly, forexample, Flux H includes 6-12 weight % glutaric acid as an activator and88-94 weight % glycerol ethoxylate as a medium-viscosity solvent. Flux Iincludes 6-12 weight % glutaric acid, 84-90 weight % glycerol ethoxylateas a medium-viscosity solvent, and 4-10 weight % multi-amines.

In one or more embodiments of the invention, molecular weights (MW) ofhigh viscosity (tacky) solvents are in the range of 1,500-12,000. Aviscous solvent with less than 1,500 MW or with greater than 12,000 doeswork properly because a low MW solvent does not provide enough tackinesswhile a high MW solvent cannot be effectively cleaned with water afterreflow. The range of viscosity of the high viscosity (tacky) solvent oractivator can include from 400 centi-Stokes (cSt) at 20 degrees Celsiusto 35,000 cSt at 20 degrees Celsius. The MWs of medium viscosity (tacky)solvents are in the range of 500-5,000 while their viscosity can be inthe range of 100-1,000 cSt. Additionally, in one or more embodiments ofthe invention, the boiling point of any flux component is greater than250 degrees Celsius, a typical reflow temperature for lead-free solders.

Further, by way of example, tackiness of flux A is in the range of250-2,000 gram force (gF), that of flux C in the range of 250-1,000 gF,and that of flux H in the range of 260-330 gF.

Also, by way of example, flux A can be prepared by dissolving glutaricacid activator in a solid state into a mixture of the medium and highviscosity solvents at 60° C. for 1-2 hours. Shaking theactivator-solvent mixture can assist in the dissolution. In an exampleof Flux A, 10 weight % (of the total composition) glutaric acid wasadded to a mixture solvent of 45 weight % glycerol ethoxylate and 45weight % ethylene glycol-propylene glycol random-copolymer. The entiremixture was heated to 60° C. for one hour and shaken. A homogeneoussolution of the flux was prepared. The measured tackiness of this fluxwas in the range of 410-440 gF.

Additionally, for example, flux C can be prepared by mixing all threecomponents, which are all liquids, at 60° C. for 1 hour. In an exampleof flux C, 20 weight % poly(ethylene glycol)-diacid, 40 weight %glycerol ethoxylate, and 40 weight % ethylene glycol-propylene glycolrandom copolymer were mixed at 60° C. for one hour to make a homogeneoussolution of the flux. The measured tackiness of this flux was in therange of 280-330 gF.

Further, for example, flux H can be prepared by dissolving 8% by weightglutaric acid in a solid state into 92% by weight glycerol ethoxylate ina liquid state. The mixture can be heated to 60° C. for one hour andshaken. A homogeneous solution of the flux is then prepared, and themeasured tackiness of this flux was in the range of 280-300 gF.

Fluxes such as described herein work in flip-chip joining of Pb-freesolders. By way of example, Flux A can be spray-applied or brushed ontoa laminate. Then, a large chip (20 millimeters (mm)×20 mm) with Pb-freesolders such as, for example, Sn/Cu or Sn/Ag/Cu was placed on thelaminate (55 mm×55 mm). The chip-laminate module went through a reflowprocess at 235-250 degrees Celsius. Flux residue, especially at thechip-laminate gap, was removed with deionized water at 40-80 degreesCelsius. All solder bumps joined to laminate capture pads.

Multi-amines in fluxes E, F and I help improve the capability of solderjoining and flux-residue cleaning, but they can also tend to diffuseinto a solder mask layer of a laminate (substrate) during reflow, andthen react with alkyl chloride impurities of the solder mask to formfree chloride ions which, in turn, cause corrosion of copper circuitry.Such a problem can be determined by humidity reliability tests such as ahighly accelerated temperature and humidity stress test. However, forexample, a flux without an organic amine, such as in one or moreembodiments of the invention, does not cause such a problem. Fluxes A,B, C, D, G and H do not contain amines. Such a problem does not occurwith a solder mask that has a glass transition temperature much higherthan the reliability test temperature. In this case, fluxes E, F and Ican also be used.

FIG. 1 is a diagram illustrating a substrate, according to an embodimentof the present invention. By way of illustration, FIG. 1 depicts asubstrate 12, a pad 14, and a ball grid array (BGA) 16. The substratecan be a laminate containing organic dielectrics, a ceramic substratemade of ceramic dielectrics, a silicon substrate, a glass substrate, afilm substrate, a print circuit board (PCB) or a semiconductor device.The pad can be a solder pad or a contact pad. A flux is applied onto thesubstrate. As described herein, basic requirements of flux can include,by way of example, the following. A flux is preferably tacky enough tohold a chip and a substrate together to overcome vibrations of areflow-oven belt and emission of vapors. Also, a solder joining shouldbe efficient, producing no non-wets. Further, a flux preferably cleansthe chip/substrate/solder joint surfaces with water, with no fluxresidue left on any of the three surfaces after cleaning.

FIG. 2 is a diagram illustrating a flux configuration, according to anembodiment of the present invention. By way of illustration, FIG. 2depicts a substrate 12, a pad 14, a ball grid array 16, a flux 20, asemiconductor device (chip) 22 and a solder ball 24. A solder ball 24can be replaced by a copper pillar capped with solder. When a substrate12 had a solder pad 14, a solder ball 24 can be replaced by a copperpillar bump or a metal (gold or copper) stud bump. A chip is to bejoined onto a substrate to which a flux is applied. A substrate can be,for example, a laminate or a ceramic substrate.

FIGS. 3A and 3B are diagrams illustrating techniques for soldering achip to a substrate, according to an embodiment of the presentinvention. By way of illustration, FIG. 3A (and, largely, FIG. 3B aswell) depicts a substrate 12, a ball grid array 16, a chip 22, a solderjoint (after reflow) 28, and a flux residue 30. Cleaning the fluxresidue gives FIG. 3B. As such, FIG. 3B depicts a chip-substrate modulefor which the flux residue is removed typically with water by sprayingwater at 40-80° C. onto the substrate and into the gap between chip andsubstrate. Underfill is then applied to the gap to maintain theintegrity of the chip-substrate module. The flux residue, if leftuncleaned, can cause a failure at the chip-underfill and/orsubstrate-underfill interface.

FIG. 4 is a diagram illustrating a flip-chip package configuration,according to an embodiment of the present invention. By way ofillustration, FIG. 4 depicts a substrate 12, a chip 22, a solder joint28, a underfill 42, a heat sink (lid) 44, a capacitor 45, a printedwiring board 46, and a BGA joint 48. The underfill not only holds thechip and the substrate together, but also prevents moisture fromdiffusing to solder joints. The heat sink dissipates heat coming fromthe chip. BGAs are joined to the large printed wiring board which isalso called mother board.

The flux composition of the present invention can be applied, forexample, to a chip cage of a substrate, where a chip will be placed,including solder pads 14 (described above in conjunction with thedescription of FIG. 2). The flux composition may also be applied tocontact pads 14 (described above in conjunction with the description ofFIG. 2). Additionally, the flux composition can also be applied to achip 22 including solder balls 24 (described above in conjunction withthe description of FIG. 2). The flux composition can also be applied forjoining of a capacitor (or resistor) 45 onto a substrate 12 as well asfor assembly of a substrate 12 on a printed wiring board 46. Applicationof the flux composition removes oxide layers out of the solder surfacesand helps achieve chip-substrate solder joining (wetting). The fluxcomposition may be applied to one or more of these structures usingconventional application devices, including, but not limited to, asyringe, a brush, a sprayer, a dipper and combinations comprising atleast one of the foregoing application devices.

FIG. 2 shows that after the flux composition of the present inventionhas been applied, where a semiconductor chip 22 is positioned relativeto substrate (for example, laminate 12) such that one or more of solderballs 24 contact one or more of the corresponding solder pads 14. Assuch, a continuous contact is established from solder balls 24 onsemiconductor chip 22 to contact pads 14 on substrate 12.

With solder balls 24 and solder pads 14 in contact with each other, thesemiconductor chip 22/substrate 12 assembly is heated to melt at least aportion of solder balls 24 and/or solder pads 14. In an exampleembodiment of the invention, heating is conducted in an oven. During theheating process, the assembly is heated to a temperature of from about25° C. to about 50° C. above the melting temperature of the solder, toreach a peak reflow temperature. This heating above the meltingtemperature of the solder helps to ensure that all of the solder reachesa reflow temperature.

For example, a eutectic solder composition, for example, one comprisingabout 37 percent Pb and about 63 percent Sn, has a melting temperatureof about 183° C. and thus the peak reflow temperature would be betweenabout 208° C. to about 233° C. Lead-free solders comprising, forexample, about 99.3 percent Sn and about 0.7 percent Cu, have a meltingtemperature of about 227° C. and those comprising, for example, about95.5 percent Sn, about 3.8 percent Ag and about 0.7 percent Cu, have amelting temperature of about 217° C. The melting temperature of acertain lead-free solder composition can be approximately 275° C. Forthese lead-free solders, the peak reflow temperature can be as high as325° C. The assembly may be kept at the peak reflow temperature for aduration of from about 90 seconds (sec)±15 sec to about 120 sec±15 sec.

The temperature is then lowered to room temperature. In an exampleembodiment, the temperature is lowered from the peak reflow temperatureat an average rate of about 0.1° C. per sec (as measured from the peakreflow temperature down to 190° C.). As a result of the heating step,solder balls 24 and solder pads 14 undergo melting to make continuousmetallurgical and electrical connections between semiconductor chip 22and substrate 12.

Residues may form on surfaces of the assembly, for example, on thesolder-joined regions. These residues can typically include metallicoxides (for example, tin oxides) and organics from flux andorganometallic compounds that are formed by the reaction between thesolder metals and flux components. Washing may be used to remove theseresidues. Washing may comprise use of water wash or first the use of anacid wash followed by a water wash.

In one or more embodiments of the invention, the continuous solderconnections between semiconductor chip 22 and substrate 12, describedabove in conjunction with the description of FIG. 3B, may then beencapsulated in, for example, a mixture of an epoxy resin and inorganicfillers, to relieve any strain which may be caused by a differencebetween the coefficient of thermal expansion (CTE) of substrate 12 andthe CTE of semiconductor chip 22. Encapsulating the connections can beaccomplished using conventional techniques.

Since Pb may pose health risks, it may be desirable to reduce oreliminate Pb from electrical components. Thus, certain solder compoundsmay contain, at most, a limited amount of Pb. In one or more embodimentsof the invention, a flux composition such as described herein may beused in conjunction with these lead-free solders, especially when themajor component of such solder compounds, as is commonly the case, isSn.

The present flux composition and solder-joining techniques may beemployed to mount a pinless chip carrier module, comprising at least onesemiconductor chip, for example, an organic module or a ceramic module,to a PCB. Mounting of such a chip carrier module may be accomplishedusing techniques similar to the mounting techniques described above.

A flux composition and solder-joining techniques of one or moreembodiments of the invention may be employed to mount other electroniccomponents, including, but not limited to, resistors and capacitors ontoa substrate. Further, while the above techniques have been described inthe context of solder-joining electrical components, it is to beunderstood that the present flux composition and techniques for the usethereof are suitable for any applications that involve flux compositionsand the use thereof in solder-joining.

FIGS. 5A and 5B are flow diagrams illustrating techniques for joiningobjects, according to an embodiment of the present invention. Step 52includes applying a flux composition to at least a portion of one ormore of the objects. Objects can include, for example, a semiconductordevice, a chip carrier module, a laminate, a silicon substrate, a glasssubstrate, a film substrate, a printed circuit board, and/or a ceramicsubstrate. The flux composition can be applied, for example, to one ormore solder regions, to one or more contact pads present on one or moreof the objects, etc. Additionally, applying the flux composition caninclude using an application technique selected from the group includingwaving, spraying, dipping, brushing, and combinations of the foregoingapplication techniques. Step 56 includes applying a flux composition anda solder compound to at least a portion of one or more of the objects.Objects can include, for example, a semiconductor device, a chip carriermodule, a laminate, a printed circuit board, and/or a ceramic substrate.

In one or more embodiments of the invention, the flux composition (whichis water soluble) includes an activator(s), a medium-viscosity solventbeing a polymer (for example, a polymer with three hydroxyl end groups),and a high-viscosity solvent being a copolymer containing a plurality offirst monomers and a plurality of second monomers. The first monomersand the second monomers can be in a random (or block) sequence to form arandom copolymer (or block copolymer). The high-viscosity solvent canalso include two hydroxyl end groups. A tackiness measure of thecomposition, for example, can be a range from 250 gram force to 2,000gram force.

The medium-viscosity solvent includes at least one of glycerolethoxylate, glycerol propoxylate, and glycerol ethoxylate-propoxylatecopolymer. The medium-viscosity solvent can contain three hydroxyl endgroups to facilitate miscibility with water and activators. Also, in oneor more embodiments of the invention, the medium-viscosity solvent has aviscosity range of 100 centi-Stokes to 1,000 centi-Stokes.

Further, the high-viscosity solvent includes at least one of ethyleneglycol-propylene glycol random copolymer, ethylene glycol-propyleneglycol-ethylene glycol block copolymer, and mixtures thereof. Thehigh-viscosity solvent can be a linear polymer to enhance viscosity andtackiness that also contains two hydroxyl end groups to facilitatemiscibility with water. Also, in one or more embodiments of theinvention, the high-viscosity solvent can have a molecular weight in arange of 1,500 to 12,000, and a viscosity range of 400 centi-Stokes to35,000 centi-Stokes. Also, in one or more embodiments of the invention,the high-viscosity solvent can include, for example, selectedcommercially available solvents including DOW Chemical's Polyox™,cellulose polymers, and UCON™ fluids.

The activator can include, for example, a diacid in a liquid state, aswell as multi-acids that contain two or more carboxylic acid groups. Theactivator can also include, for example, keto acid, which can beselected from levulinic acid, acetyl butyric acid, 2-acetylbenzoic acid,2-acetyloxybenzoic acid, 2-ketobutyric acid, acetoxyacetic acid, andpyruvic acid or mixtures thereof. Further, in one or more embodiments ofthe invention, the activator is soluble in both the solvents and water,and it has a boiling point of greater than 250 degrees Celsius. Thesolvents can each also have a boiling point of greater than 250 degreesCelsius.

Additionally, in one or more embodiments of the invention, the fluxcomposition can include one or more highly tacky multi-acids. The tackyactivator can be a liquid that has a viscosity range of 200 centi-Stokesto 10,000 centi-Stokes, and a boiling point of greater than 250 degreesCelsius.

Also, in one or more embodiments of the invention, the flux compositioncan include one or more multi-amines such astetrakis(4-hydroxypropyl)ethylenediamine (Quadrol®) andtetrakis(4-hydroxyethyl)ethylenediamine.

Further, in one or more embodiments of the invention, the compositionincludes from about five percent by weight to about twenty percent byweight of the activator, from about fifteen percent by weight to aboutseventy-five percent by weight of the medium viscosity solvent, and fromabout twenty percent by weight to about eighty percent by weight of thehigh viscosity solvent, based on total weight of the composition. One ormore embodiments of the invention can also include from about fourpercent by weight to about ten percent by weight of multi-amines, basedon total weight of the composition.

Steps 54 and 58 include joining the objects. Joining the objects caninclude introducing the objects to heat (for example, heat that includesa range from about 220° C. to about 325° C.). The techniques depicted inFIGS. 5A and 5B can additionally include removing flux residue formedduring joining. Removing flux residue can include, for example, usingwater to remove flux residue.

One or more embodiments of the invention can also include a fluxcomposition that includes an activator, and a high-viscosity solvent,wherein the high-viscosity solvent comprises a copolymer containing aplurality of first monomers and a plurality of second monomers.

Additionally, one or more embodiments of the invention include a fluxcomposition that includes one or more activators from the selecteddiacids (for example, an activator of 6-12 percent by weight of glutaricacid, pimelic acid, tartaric acid, or mixtures thereof, based on totalweight of the composition), and a medium-viscosity solvent (for example,of 88-94 percent by weight of based on total weight of the composition),wherein the medium-viscosity solvent comprises a polymer with hydroxylend groups. The medium-viscosity solvent can include, for example, atleast one of glycerol ethoxylate, glycerol propoxylate, a glycerolethoxylate-propoxylate copolymer and mixtures thereof. By way ofexample, such a flux composition can include an activator of glutaricacid and a medium-viscosity solvent of glycerol ethoxylate.Additionally, such a flux composition can also include from about fourpercent by weight to about ten percent by weight of multi-amines, basedon total weight of the composition.

Further, one or more embodiments of the invention include a fluxcomposition that includes an activator in a liquid state (for example, adiacid or multiacid such as poly(ethylene glycol)-diacid), and amedium-viscosity solvent, wherein the medium-viscosity solvent comprisesa polymer with hydroxyl end groups. The medium-viscosity solvent caninclude, for example, at least one of glycerol ethoxylate, glycerolpropoxylate, a glycerol ethoxylate-propoxylate copolymer and mixturesthereof. Also, such a flux composition can include from about fourpercent by weight to about ten percent by weight of multi-amines, basedon total weight of the composition.

At least one embodiment of the invention may provide one or morebeneficial effects, such as, for example, a flux composition, theproperty of which includes flip-chip solder joining capability, hightackiness, water cleaning capability, and no boiling at reflow.Additionally, at least one embodiment of the invention also providesbeneficial effects such as, for example, providing a flux with increasedtackiness while the flux maintains other standard flux properties (forexample, water solubility).

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

What is claimed is:
 1. A soldering method for joining objects,comprising the steps of: generating a flux composition by: combining, atapproximately sixty degrees Celsius for at least approximately one hour,(i) about five percent by weight to about twenty percent by weight of anactivator consisting of a mixture of 2-acetyloxybenzoic acid, acetylbutyric acid, levulinic acid, 2-acetylbenzoic acid, 2-ketobutyric acid,acetoxyacetic acid, and pyruvic acid, (ii) about fifteen percent byweight to about seventy-five percent by weight of a medium-viscositysolvent being a polymer in a liquid state, wherein the medium-viscositysolvent consists of a mixture of glycerol ethoxylate, glycerolpropoxylate, and a glycerol ethoxylate-propoxylate copolymer, (iii)about twenty percent by weight to about eighty percent by weight of ahigh-viscosity solvent with hydroxyl end groups, wherein saidhigh-viscosity solvent comprises a random copolymer containing aplurality of first monomers and a plurality of second monomers combinedin a random sequence to form the random copolymer, and wherein saidhigh-viscosity solvent is in a liquid state at room temperature, and(iv) about four percent by weight to about ten percent by weight ofmulti-amines, wherein the multi-amines comprisestetrakis(4-hydroxyethyl)ethylenediamine; applying the flux compositionto at least a portion of one or more of the objects; and joining theobjects.
 2. The method of claim 1, further comprising applying a soldercompound with the flux composition to at least a portion of one or moreof the objects.
 3. The method of claim 1, wherein joining the objectscomprises introducing the objects to heat, wherein the heat comprises arange from about 220° C. to about 325° C.
 4. The method of claim 1,further comprising removing flux residue formed during joining, whereinremoving flux residue formed during joining comprises using water toremove flux residue.
 5. A soldering method for joining objects,comprising the steps of: generating a flux composition by: combining, atapproximately sixty degrees Celsius for at least approximately one hour,(i) about six percent by weight to about twelve percent by weight of anactivator in a liquid state at room temperature that is soluble in boththe solvents and water, wherein the activator consists of a mixture of2-acetyloxybenzoic acid, acetyl butyric acid, levulinic acid,2-acetylbenzoic acid, 2-ketobutyric acid, acetoxyacetic acid, andpyruvic acid, (ii) about eighty-eight percent by weight to aboutninety-four percent by weight of a medium-viscosity solvent, wherein themedium-viscosity solvent consists of a mixture of glycerol ethoxylate,glycerol propoxylate, and a glycerol ethoxylate-propoxylate copolymer,and (iii) about four percent by weight to about ten percent by weight ofmulti-amines, wherein the multi-amines comprisestetrakis(4-hydroxyethyl)ethylenediamine; applying the flux compositionto at least a portion of one or more of the objects; and joining theobjects.
 6. The method of claim 5, further comprising applying a soldercompound with the flux composition to at least a portion of one or moreof the objects.
 7. The method of claim 5, wherein joining the objectscomprises introducing the objects to heat, wherein the heat comprises arange from about 220° C. to about 325° C.
 8. The method of claim 5,further comprising removing flux residue formed during joining, whereinremoving flux residue formed during joining comprises using water toremove flux residue.